The work proposed in this application will be a continuation of current research and will be directed towards two chief objectives: First, synthesis of modified tris-amidines as inhibitors of pancreatic kallikrein, and second, clarification of the role of various proteases in the regulation of pancreatic exocrine secretion. In the last year we have shown that a novel group of compounds, i.e., tris-amidinophenoxy derivatives of mesitylene, are strong, preferential inhibitors of pancreatic kallikrein. These compounds make contact with the enzyme at the active center, and it is assumed that one of the cationic moieties enters the specificity pocket while the other two are positioned in the binding groove to the right and to the left of the pocket, respectively. Enzyme-inhibitor interaction is greatly influenced by certain changes in the inhibitor structure, and it is our plan to achieve optimal binding conditions by examining the effect of a number of molecular modifications. This will involve substitution of the amidino group, altering the chain length, and introduction of heterocycles. In this manner, we hope to increase not only the potency of the compounds, but also their selectivity especially for pancreatic kallikrein. Increased specificity of the compounds will be of crucial importance for the second part of the investigation, i.e., the study of the "trypsin" feedback control of pancreatic enzyme output. With the help of the inhibitors we intend to identify the pancreatic protease whose presence in the small intestine is responsible for the suppression of secretory activity. Once this is accomplished, the purified enzyme is to be used to pinpoint its role in the feedback mechanism. BIBLIOGRAPHIC REFERENCES: Geratz, J.D., Cheng, M.C.-F., and Tidwell, R.R.: Novel bis (benzamidino) compounds with an aromatic central link. Inhibitors of thrombin, pancreatic kallikrein, trypsin, and complement. J. Med. Chem. 19: 634, 1976. Tidwell, R.R., Fox, L.L. and Geratz, J.D.: Aromatic tris-amidines. A new class of highly active inhibitors of trypsin-like proteases. Biochim. Biophys. Acta 445: 729, 1976.